1. Field of the Invention
This invention relates to privacy transmission systems in which speech or other messages are scrambled prior to transmission and unscrambled upon reception.
2. Description of the Prior Art
The prior art discloses numerous privacy transmission systems in which speech or other messages are scrambled prior to transmission and unscrambled upon reception. The scrambling and unscrambling processes frequently operate on the frequency and/or time parameters of such messages. Whether it is these or other parameters, it is of course essential that the two processes be performed in synchronism so that the original message is recovered. Furthermore, for reliable privacy it is also essential that the scrambling and unscrambling processes limit the use of a particular key (i.e., code or format) so that an unauthorized receiver cannot use known techniques to unscramble a portion of the message.
U.S. Pat. No. 2,405,500 issued to G. Guanella on Aug. 6, 1946 discloses a privacy system conceived to achieve the above-mentioned results. This system uses a control signal to control the scrambling of a message and then transmits the control signal along with the scrambled message. When received, the control signal is used to control the unscrambling process. The control signal in this system may however be extracted at any point along the message transmission path by an unauthorized receiver. When the unauthorized receiver has access to the keys available for scrambling and unscrambling, the extracted control signal can be used to unscramble the message in the same manner as the authorized receiver.